Event Handling Details

The following modifications are required for the generic applet to include the event handling from user actions. For now, we will proceed with the assumption that the applet itself will handle the events.

  • Modifying the applet declaration

    • The declaration of the applet seen so far had the following syntax: 

     public class TestApplet extends JApplet
    For the applet to behave as the event handler, the declation is modified as follows: 
     public class TestApplet extends JApplet 
                    implements <one or more Listeners>
    For example, to respond to button events, the applet declaration would be: 
     public class TestApplet extends JApplet 
                    implements ActionListener
    To respond to slider (scroll) events, the applet declaration would be: 
     public class TestApplet extends JApplet 
                    implements ChangeListener
    To respond to mouse motion events, the applet declaration would be: 
     public class TestApplet extends JApplet 
                    implements MouseMotionListener
    In case the applet needs to respond to multiple types of events, say buttons and mouse events, the applet declaration implements all the required listeners: 
     public class TestApplet extends JApplet 
                    implements ActionListener, 
                    MouseMotionListener
  • Adding listeners to the components

    • The next step, once the GUI components (buttons, text fields, etc.) are created, is to add the appropriate listeners. The following are some examples of listeners for buttons, slider, and for the applet itself. 
     button.addActionListener(this); 
     slider.addChangeListener(this); 
     this.addMouseMotionListener(this);
    In the first case, an action listener is added to variable named button (of type JButton). In the second case, a change listener is added to the variable named slider (of type JSlider). In the last case, the mouse motion listener is added to the applet itself, refered to by the variable this. The argument to the listener in all the above cases is the applet itself (hence the value this).
  • Event handling method definitions 

    • Depending upon the listener implemented by the applet, the code for the corresponding event handling methods need to be implemented by the applet. For button event handling, the following method definition is required as part of the applet code: 
               public void actionPerformed(ActionEvent e) {
       		// code to respond to the button
           }
    Whenever a button click occurs, the above method is automatically invoked. The argument for the method details the type of the event and the details asscoiated with the event. When there is only one button as in the previous program, we obviously know the source of the event.